The present invention relates to transmission of data on a subscriber loop in a public network such as, for example, a telephone network. More specifically, the present invention provides an improvement of standard single-line digital subscriber line (SDSL) technology.
A wide variety of technologies and transmission standards have been developed for transmission of data via currently existing public network resources. The term “digital subscriber line” refers to a modem or modem pair connected by one or more twisted pairs having a specific data frame format and associated transmission rate. The first digital subscriber line technology, referred to as IDSL, corresponds to what is also known as basic rate ISDN. IDSL technology transmits duplex data at 144 kbps over copper lines using a 2B1Q modulation scheme. The modems multiplex and demultiplex the data stream into two B channels (64 kbps each) and a D channel (16 kbps) as described in ANSI T1.601, the entirety of which is incorporated herein by reference for all purposes.
High data rate digital subscriber lines (HDSL) are related to the earlier IDSL using the same modulation scheme to transmit data at the T1 data rate over two twisted pairs as described in ANSI Committee T1 TR-28 and ETR 152, the entireties of which are incorporated herein by reference for all purposes. A single line digital subscriber line (SDSL) is a single pair version of HDSL, i.e., transmitting data at one-half the T1 data rate, i.e., 768 kbps, over a single twisted pair.
For both HDSL and SDSL and as shown in FIG. 1, data are organized into 6 ms frames 102 comprising alternating overhead and payload sections 104 and 106. The four payload sections 106 each include twelve 97-bit payload blocks 108, 96 bits (110) of which are data and one bit (112) of which is known as the F/Z bit. This works out to the well known SDSL data rate of 768 kbps. Overhead sections 104 along with bits 112 represent an additional 16 kbps for an actual line rate of 784 kbps.
FIG. 2 is a simplified block diagram of a standard SDSL 202 connecting a central office or public branch exchange (represented by modem 204) and a subscriber premises (represented by modem 206). The data to be transmitted enters framing circuitry 208 of modem 204 at the raw data rate of 768 kbps. Framing circuitry 208 organizes the incoming data stream into the 6 ms frames described above with reference to FIG. 1 using a 768 kHz oscillator 210 and frame overhead data generated at 16 kbps. The framed data are then sent to bit pump 212 where, using a 784 kHz oscillator 214, they are encoded according to the 2B1Q modulation scheme and transmitted via twisted pair 216 to the subscriber premises as represented by modem 206.
At the subscriber premises, the modulated framed data are received at the rate of 784 kbps and demodulated by bit pump 218 which is clocked by a 784 kHz oscillator 220. The demodulated data are then received by framing circuitry 222 which strips off the 16 kbps frame overhead data and decomposes the 6 ms HDSL frames into a 768 kbps data stream. Framing circuitry 222 is clocked by a phase-locked loop (PLL) recovered clock (PLL circuitry 224) derived from the incoming data stream.
The F/Z bit in each payload block (field 112 of FIG. 1) is reserved for use with standard T1 and E1 for loop identification and out-of-band signaling for the transmission of special alarm codes which are only applicable to T1 and E1 transmissions. When standard framing circuitry receives a standard frame, the F/Z bit information is written to an F/Z bit register associated with the framing circuitry. If the transmission is a T1 or E1 transmission, the F/Z bit data are then used in accordance with their intended purpose according to well known techniques. However, when a data frame is on a network link which uses only SDSL technology and does not interface with T1 or E1 technology, the F/Z bit register and the data stored therein are ignored representing 8 kbps of wasted bandwidth. This may seem a relatively small amount of bandwidth with reference to the standard rate of 768 kbps. However, when viewed relative to the subject matter described in commonly assigned, copending U.S. Patent Application Ser. No. 09/107,840 for METHODS AND APPARATUS FOR TRANSMITTING DATA IN A PUBLIC NETWORK filed simultaneously herewith, the entire specification of which is incorporated herein by reference for all purposes, such a small bandwidth increase becomes more important. That is, the above-referenced copending application describes methods and apparatus by which the current HDSL data transmission infrastructure may be leveraged to provide lower effective data rates than the standard 768 kbps without sacrificing the advantages of the standard frame format. At the lower transmission rates made possible by those techniques, an extra 8 kbps could prove extremely valuable.
It is therefore desirable to provide data transmission techniques which avoid wasting this valuable data bandwidth on SDSL transmission links while retaining the compatibility advantages of the standard frame.